1. Field of the Invention
This invention relates to a floatation apparatus and more particularly to a continuously-filled buoy for maintaining an object at a near constant depth below the surface of a body of water.
2. Background of the Invention
In marine seismic surveying, to obtain geophysical information relating to the substrata located below the sea bottom, acoustic transmitters adapted to produce pressure pulses or shock waves under water are towed beneath the water surface behind a marine vessel. The shock waves propagate into the substrata beneath the sea where they are refracted and reflected back to the sea. The returning shock waves are detected by sensors (usually hydrophones) and the useful data contained in the signals produced by the sensors is processed to determine the geophysical structure of the substrata.
Air guns or gas guns are frequently used as acoustic transmitters. During seismic surveying, several air guns are placed in spaced relation to each other in a subarray. One or more air gun subarrays are towed beneath the sea surface to produce desired pressure pulses. The pressure pulse characteristics of each air gun along with its relationship to the other guns in the subarray determine the characteristics of the pressure pulses produced, such as frequency, bubble ratio and amplitude.
In order to produce and transmit accurate pressure pulses having known characteristics, it is important that the air gun subarray is maintained at a constant depth below the water surface and in a near straight line horizontal position. Air gun subarrays presently in use are generally more than fifty (50) feet long and weigh several hundred pounds. To keep such air gun subarrays at a constant depth below the water surface, it is a common practice in the art of seismic surveying to pivotly attach a single buoy along the length of the air gun subarray by means of parallel links. The buoy maintains the air gun subarray at or near a constant depth below the water surface when the subarray and buoy are towed behind a vessel.
U.S. Pat. No. 4,686,660 to Gjestrum et al. issued Aug. 11, 1987, discloses one such buoy, which contains several discrete float chambers disposed in longitudinal spaced relation inside a tubular sleeve member. In the alternative, the discrete float chambers may be secured together longitudinally or they may be connectable lengths of a sleeve. However, regardless of the manner in which the discrete float chambers are used to form the buoy, they are not in fluid (generally air) communication with each other and thus, must be pressurized with air prior to use. Once such a buoy is deployed, no means exist to refill any of the chambers should an air leak occurs without shutting down the entire operation and pulling the subarray and the buoy onto the vessel for repair or replacement. Such prior art pre-airfilled discrete chambers have frequently failed even due to minor air leaks over a period of time causing a portion of the air gun subarray to sag, which distorts the relative positioning of the air guns thereby causing distorted pressure pulses to be produced which, of course, is highly undesirable.
The equipment utilized for seismic surveying includes, among other things, air gun subarrays, seismic cables, data acquisition and processing equipment, and a marine vessel. The total cost of the entire equipment can easily exceed fifteen (15) million dollars. Due to the high cost of the equipment and the logistics of conducting seismic surveying offshore, the surveying activity is usually performed around the clock for several days at a time, except for the time it takes to change crews between working shifts or due to equipment failure. Since there exist no means to replenish the air in the discrete chambers should a leak occur, any failure relating to such a buoy will either require shutting down the surveying activity to pull the buoy and the air gun on the vessel or result in obtaining inaccurate seismic data. Neither of these alternatives is, of course, acceptable. Therefore, it is highly desirable to have a reliable buoy or floatation system for use in marine seismic surveying whose performance is unaffected by air leaks.
The present invention addresses the problems associated with the prior art floatation devices and provides a floatation device which is substantially unaffected by air leaks.